1. Field of the Invention
The present invention relates to a hermetic terminal cover for use in, such as, a closed-type compressor. The present invention further relates to a closed-type compressor employing such a hermetic terminal cover therein.
2. Description of the Prior Art
In a closed-type compressor or the like, a hermetic terminal is arranged at a portion of a sealed case of the compressor for feeding power to an internal electric motor from the exterior thereof. The hermetic terminal includes a metallic base having tubular sections and terminal pins made of an alloy of, such as, iron, nickel and cobalt which respectively pass through the centers of the tubular sections with respective insulators hermetically interposed between the terminal pins and the tubular sections. Since the hermetic terminal is generally exposed to refrigerant and oil in the sealed case of the compressor, each insulator is required to have a high refrigerant resistance and a high oil resistance, and further to firmly support the terminal pin. In order to satisfy all of these requirements, the insulator made of glass has been generally utilized. The hermetic terminal of this type is disclosed, such as, in Japanese Second (examined) Utility Model Publication No. 56-16777.
FIG. 12 corresponds to FIG. 2 of this publication. In FIG. 12, a hermetic terminal generally designated by a reference numeral 100 includes three continuous terminal strips 101. Each terminal strip 101 includes a tubular section 105 and a terminal pin 107 which is fixedly received through the tubular section 105 with an insulator 106 of glass interposed therebetween. Specifically, a dish-shaped metallic base 103 is formed with three through holes 102 each of which is defined by the tubular section 105 of the metallic base 103. Each terminal pin 107 fixedly passes through a center of the through hole 102 in hermetic and electrically insulated manners by means of the glass insulator 106 interposed between the terminal pin 107 and an inner periphery of the tubular section 105. A reference numeral 109 denotes a sealed case of the closed-type compressor, and the hermetic terminal 100 is fixed to the sealed case 109 with its outer periphery welded thereto.
The refrigerant and the oil in the sealed case 109 include therein metallic powders, such as, iron powders and copper powders which have been generated, for example, at the time of assembling the sealed case by welding or due to abrasion produced at the time of sliding movement of an internal piston. These metallic powders are carried by the refrigerant to various places in the sealed case 109 to be adhered or attached thereto. Problem is raised when these metallic powders are attached to an exposed surface of each of the insulators 106 of the hermetic terminal 100. Specifically, when an amount of the metallic powders adhered to the surface of each insulator 106 is increased and a large voltage is impressed to the terminal pin 107 at the time of, for example, restarting the compressor, sparks are liable to be generated between the terminal pin 107 and the tubular section 105 through the attached metallic powders, leading to accidents, such as, the leak and the breakage of the hermetic terminal 100.
In order to solve the above-noted problem, there has been proposed an improved hermetic terminal as disclosed in Japanese First (unexamined) Patent Publication No. 62-271975which will be explained with reference to FIGS. 13 and 14.
In FIGS. 13 and 14, a reference numeral 111 denotes a disc-shaped protection cover formed, by drawing, of polyethylene terephthalate (hereinafter referred to as "PET") of about 250 to 300 pm. The protection cover 111 has an annular folded portion 112 at its outer peripheral edge, three stepped tubular portions 113 and a center small hole 114. Each of the stepped tubular portions 113 includes an annular bulged portion 116 or an annular step, a large-diameter opening 117 and a small-diameter opening 118.
Dimensions of the relevant portions of the protection cover 111 are set as in the following manner:
An outer diameter of the folded portion 112 is set slightly larger than an inner diameter of the metallic base 103. An inner diameter of the bulged portion 116 is set slightly smaller than an outer diameter of the tubular section 105 of the metallic base 103. A diameter of the large-diameter opening 117 is set larger than the tubular section 105 of the metallic base 103. A diameter of the small-diameter opening 118 is set slightly smaller than an outer diameter of the terminal pin 107. As shown in FIG. 14, the protection cover 111 is mounted onto the hermetic terminal 100 with each terminal pin 107 press-fitly passing through the small-diameter opening 118 of the protection cover 111 and with the folded portion 112 of the protection cover 111 press-fitly engaged with the inner periphery of the metallic base 103.
As appreciated from the foregoing dimensional relationship, fluid-tight conditions are provided between a wall of each small-diameter opening 118 and the corresponding terminal pin 107, between each bulged portion 116 and the corresponding tubular section 105 and between the folded portion 112 and the inner periphery of the metallic base 103. Further, a gap or clearance 120 is provided between the surface of each insulator 106 and the corresponding tubular portion 113 of the protection cover 111. Accordingly, the protection cover 111 of PET hermetically covers the exposed surface of the insulator 106, and in addition, provides the gap 120 therebetween. As appreciated, electric force lines are the largest in number at the surface of the insulator 106, i.e. an electric field is the strongest therearound. Accordingly, by spacing the protection cover 111 from the surface of the insulator 106 by the gap 120, an attracting force of static electricity induced at the surface of the insulator 106 is largely weakened at an outer surface of the protection cover 111 so that it is possible to largely reduce an amount of the metallic powders to be attracted and adhered to the outer surface of the protection cover 111 around the tubular section 105 of the metallic base 103. As a result, sparks can be prevented from generating between the terminal pins 107 and the tubular sections 105 of the metallic base 103.
In this prior art structure, however, a pressure within each gap 120 is determined by an ambient atmospheric pressure at the time of mounting the protection cover 111 to the hermetic terminal 100. Since the protection cover 111 is thin, i.e. it is formed of a thin plate of a synthetic resin, such as, PET, and further, a pressure in the sealed case 109 is variable according to operating and non-operating conditions of the compressor, a magnitude of the gap 120 should be set rather small so as to prevent the protection cover 111 from displacing due to the variation of the pressure in the sealed case 109. As a result, it is practically impossible to largely space the protection cover 111 from the surfaces of the insulators 106 where the number of electric force lines is the largest, so that the attraction of the metallic powders to the protection cover 111 due to the induced static electricity can not be prevented effectively or sufficiently. This leads to generation of the sparks between the terminal pins 107 and the tubular sections 105 of the metallic base 103 through the adhered metallic powders on the surface of the protection cover 111 so that the leak or the like is resulted.
Further, since the prior art protection cover 111 is made of the thin plate having a small heat capacity, it is possible that portions of the protection cover 111 heated due to the generated sparks may be lost to cause the insulator 106 to be exposed, leading to accidents, such as, the leak and the breakage of the hermetic terminal.
It is also to be appreciated that, since the prior art protection cover 111 is thin as a whole, it is difficult to handle it, meaning that mounting it onto the hermetic terminal 100 can not be done smoothly.
Further, since the prior art protection cover 111 is designed to entirely cover a bottom of the dish-shaped metallic base 103 including the three continuous terminal strips 101, air is enclosed or caught between the protection cover 111 and the bottom of the metallic base 103 when the protection cover 111 is mounted to the hermetic terminal 100, so that the tightness therebetween becomes poor. For solving this problem, the small hole 114 is formed at the center of the protection cover 111 for an air vent so as to prevent the air from remaining between the protection cover 111 and the bottom of the metallic base 103. However, since the protection cover 111 is so thin, it can not be avoided that cockles or wrinkles are generated to catch the air therein. As a result, it becomes necessary to move the caught air toward the air vent hole 114 by smoothing the wrinkles by, such as, operator's hands after having mounted the protection cover 111 onto the metallic base 103, so as to release the air little by little through the air vent hole 114, which is complicated and troublesome.
Still further, since each of the stepped tubular portions 113 (hereinafter also referred to as "the protection caps") of the protection cover 111 is continuous with a planar base portion of the protection cover 111 via an annular folded portion or the like, a positional freedom of each protection cap is strictly limited, i.e. a degree of the positional freedom of each protection cap is very small. On the other hand, in the hermetic terminal 100, it is likely to happen that the terminal pin 107 is not arranged straight but bent, or the terminal pins 107 are not arranged in parallel with each other, i.e. a degree of parallelization is poor. Accordingly, when mounting the prior art protection cover 111 onto the hermetic terminal having such an irregular terminal pin, the assembling operation becomes quite troublesome due to the very small degree of the positional freedom of the protection caps 113 of the protection cover 111.
In addition, when forcibly mounting the protection cover 111 to such a hermetic terminal 100, an unexpected excessive force is applied to one or more of the small-diameter openings 118 for receiving the terminal pin or pins 107 therethrough so that the small-diameter opening 118 is forced to be permanently enlarged. As a result, a gap or clearance is generated between the terminal pin 107 and the corresponding wall defining the small-diameter opening 118, resulting in invasion of the metallic powders therethrough. It is to be appreciated that, even when the terminal pins 107 are straight and arranged in parallel with each other, there still remains a problem of the invasion of the metallic powders through such a clearance to cause a short circuit due to the fact that no particular measures for ensuring the tightness between the terminal pins 107 and the protection cover 111 are provided.
In order to avoid the expansion or enlargement of the small-diameter opening 118 as described above, it is effective to apply a force in advance to expand a portion, other than the protection caps, of the protection cover 111 which will contact the bottom of the metallic base 103 and then to mount the protection cover 111 onto the hermetic terminal 100. Although this facilitates the assembling operation, there still remains a problem of generation of wrinkles at the expanded portion of the protection cover 111 after the protection cover has been mounted onto the hermetic terminal 100.